Previously we investigated the effects of membrane-spanning peptides L24 with a sequence KKL24KK and LA12 with a sequence KK(LA)12KK on POPC membranes. We used PC-spin labels with nitroxide moiety located at the 5-, 7-, 10-, 12-, and 16-positions Both peptides ordered the lipid bilayer, decreased motional freedom of alkyl chains, and significantly increased the hydrophobicity of the membrane interior. This year we confirmed our previous results by studying the accessibility of ferricyanide ion dissolved in water into POPC-peptide membranes. In the presence of peptide the level of penetration of ferricyanide decreased significantly. These results obtained in fluid-phase membranes are consistent with previously obtained at -150C hydrophobicity profiles. Our studies give insight to protein-protein interaction in the membrane, and folding of membrane proteins in the membrane as they relate to the peptide structure, surface roughness, and structural conformability of two peptide drains or of a peptide and lipid when they are adjacently placed in the membrane.